1. Field of the Invention
The present invention relates to a photosensitive resin composition and an optical material using the same. More specifically, it also relates to a refractive index periodical structure, and a process for preparing the same, requiring a refractive index of at least 1.7, which is used in the field of photonics, electromagnetic wave technology, micromachine technology, microreactor technology, and the like.
2. Description of Related Art
A refractive index periodical structure having a periodical distribution of a refractive index shows a diffraction/interference action to an electromagnetic wave, and prohibits propagation of the electromagnetic wave within a specific frequency region. This phenomenon corresponds to a band structure of electrons in a semiconductor crystal. Such a refractive index periodical structure is generally called a photonic crystal. And, a frequency band region in which propagation is prohibited is called a photonic band gap.
IT industries flourishing at the end of the 20th century have been supported by electronics technology based on semiconductor materials which control electrons, but are now faced with an essentially critical situation regarding technology. To further develop the technology in the 21st century, it is indispensable to transfer the technological viewpoint to a photonics technology to overcome the limitations of the electronics technology.
Photonic crystals can control an electromagnetic wave; thereby obtaining a position to be a key material in the photonics technology similar to the position of semiconductor material in the electronics technology. Photonic crystals are expected to be important elements for realizing optical devices of the next generation such as ultra-high efficiency lasers, microminiature optical integrated circuits, and other useful devices.
In order for the photonic crystal to function effectively, it is necessary to control the periodical structure with a space scale that is the same as the wavelength of an electromagnetic wave, and obtain a predetermined value for a refractive index ratio of a high refractive index phase and a low refractive index phase. A minimum or lowest refractive index ratio to be required may vary depending on the form of the periodical structure, and a larger value is generally preferred.
In the field of photonics, an object generally has a wavelength region from a visible light region to a near infrared region. Therefore, photonic crystals are generally prepared having a period from a submicron to a micron order. As a method to realize this, for example, a method has been disclosed by Lin et al. for preparing woodpile (blocks) state photonic crystals in which square pieces made of Si have been integrated with an interval of several microns by making full use of the semiconductor fine processing technology {Nature, Vol. 394, pp. 251-253 (1998)}. Also, a wafer fusion technique has been disclosed by Noda et al. as a method for preparing woodpile state photonic crystals in which square pieces made of GaAs or InP have been integrated with an interval of several microns {App. Phys. Lett., Vol. 75, pp. 905-907 (1999)}. Moreover, Kawakami et al. succeeded in preparing submicron order photonic crystals having a specific three-dimensional periodical structure comprising Si and SiO2 using an original bias sputter build-up/etching method called a self-cloning method {Electron. Lett., Vol. 33, pp. 1260-1261 (1997)}. Furthermore, Vos et al. prepared submicron order inverse opal type photonic crystals by building up titania in a space of an opal structure due to the self-assembling of monodispersed fine particles made of polystyrene according to the sol-gel method, and removing the polystyrene fine particles as molds by calcination simultaneously with calcination of the titania {Science, Vol. 281, pp. 802-804 (1998)}. Also, Misawa et al. prepared submicron order woodpile state photonic crystals comprising a photosetting resin by a two-photon absorption lithography process {Appl. Phys. Lett., Vol. 74, pp. 786-788 (1999)}.
However, the above-identified methods each have their problems. The method of Lin et al. comprises many steps using complicated semiconductor fine processing techniques. The method requires the use of large sized apparatuses resulting in low productivity, high costs, and the like, and not many types of materials can be applied to the method. Thus, it cannot be said to be a general method. The method of Noda et al. is an extremely excellent method since materials to be applied to the method are abundant and flexibility and the structure is large. However, extremely severe conditions such as heating at 700° C. under a hydrogen atmosphere have been employed to carry out wafer fusion, resulting in safety concerns associated with this method. The method of Kawakami et al. is also an extremely excellent method since materials to be applied to the method are abundant and productivity is high. However, it involves a serious problem that only a specific structure can be prepared and it cannot be applied to general use.
Opal type and inverse opal type photonic crystals can be extremely easily prepared, and have been widely used in research activities of laboratories. However, these crystals have a low flexibility in structure, so that a certain breakthrough in a preparation method would be indispensable to make a device. Also, from theoretical calculation, in an opal type and an inverse opal type photonic crystals, it is expected that refractive index conditions required to form a complete photonic band gap are extremely severe compared to those required in the woodpile state photonic crystals. Thus, they are disadvantageous from the point of flexibility in selecting materials. Also, in the inverse opal type photonic crystals, it is necessary to fill a material having a high refractive index into gaps of an opal mold.
However, this produces problems since it is difficult to fill the material into fine three-dimensional gaps uniformly and molds deform during the accompanying filling.
As a method of preparing photonic crystals using a photocuring resin, it has been proposed to use a usual optical modeling method in addition to the method using the above-mentioned two-photon absorption lithography. In this method, a fine structure can be easily obtained, but the refractive index of the resin is low, at most 1.6 or so, so there is a problem in that a large refractive index ratio cannot be obtained.